1. Field of the Invention
The present invention relates to a process for preparing a diphenyl sulfonetetracarboxylic acid.
Diphenyl sulfonetetracarboxylic acids have wide applications, for example, as materials for preparation of polyimide, polyamide-imide, polyester-imide or like resins, curing agents for epoxy resin, plasticizers, lubricants, agricultural chemicals, dyes or the like. An all-aromatic polyimide prepared by reaction of a dianhydride of said tetracarboxylic acid with an aromatic diamine is attracting attention in the fields of electrical and electronic technologies, aircraft, spacecraft, automobiles, etc. because of its high heat resistance and its excellent mechanical and electrical characteristics.
2. Prior Art
It is known to produce 3,3',4,4'-diphenyl sulfonetetracarboxylic acid by oxidizing 3,3', 4,4'-tetramethyldiphenyl sulfone with a molecular oxygen (O.sub.2). For example, an oxidation method (method 1) is known which is conducted in a solvent mixture of acetic acid and haloacetic acid such as trifluoroacetic acid, trichloroacetic acid or the like in the presence of a cobalt-manganese-bromine compound type catalyst wherein the cobalt metal and manganese metal are used in equimolar ratio [Khim. Prom. (Moscow), (5), 393 (1974)]. This method requires expensive and highly corrosive trifluoroacetic acid or trichloroacetic acid as the solvent component, hence commercially disadvantageous.
Another known method (method 2) employs as catalyst a bromine compound and an equimolar mixture of at least three IV-group metals of the periodic table, such as cobalt-manganese-chromium or cobalt-manganese-nickel and uses acetic acid as the solvent (U.S.S.R. Patent No. 422,730). This method gives a reaction product of undesirable coloration with a low purity in a low yield because the reaction is not completed.
It should be noted that diphenyl sulfone-tetracarboxylic acids to be used for the preparation of polyimide, especially pale-colored polyimide resin, must have a high purity and a low degree of coloration. The product resulting from such incomplete reaction as above contains impurities such as diphenyl sulfonetricarboxylic acid and diphenyl sulfonedicarboxylic acid and structurally unidentified various colored materials. Since these impurities are close in properties to the desired diphenyl sulfonetetracarboxylic acid, great difficulty is encountered in purifying the diphenyl sulfoneteracarboxylic acid by removal of the impurities. Consequently it is essential that the oxidation reaction give a high-purity, pale-colored product in order to satisfy the requirements necessary for use as a starting material for preparing polyimide. The above-mentioned method 2 remains to be improved in this respect.